1. Field of the Invention
The present invention relates to a process for obtaining iron or iron-based powders by organic liquid phase precipitation. The powders obtained are in finely divided form and are constituted by size-homogeneous, submicronic particles.
2. Discussion of the Background
There are considerable industrial needs for iron-based, fine powders, either as precursors for the production of mechanical parts by sintering, or for the production of composite materials, or for use as such for electrostatic reprography.
In addition, ferromagnetic metals are used in powder form, as a function of the chemical composition and morphology of the particles, for magnetic reading or recording systems, as well as in granular materials having high microwave frequency magnetic losses.
Numerous processes are known for producing iron or iron-based powders and they call on various procedures. Among the methods for producing iron-based metal powders, two major classes must be differentiated, i.e. physical and chemical methods.
Physical methods, whose two main procedures involve grinding and atomizing, consist of a morphological transformation of iron already in metallic form. In most cases, the powders obtained by these processes are formed by particles having an average size exceeding about 10 .mu.m and the particles have a random shape. Although atomization has the advantage compared with grinding of, under certain conditions, giving spherical powders, in both cases there is a very considerable size distribution. Such processes are generally unsuitable for producing monodisperse powders.
Chemical methods involve a chemical transformation of a precursor into a metal powder and can take place in the solid, liquid or gaseous phase.
The solid or gaseous phase methods give powders with a high chemical purity, but generally with a wide particle size dispersion.
On excepting the electrochemical method, which consists of reducing iron salts in solution in contact with a rotary electrode by imposing a potential difference and which gives powders having a high chemical purity, but with no control concerning the shape and size of the particles, liquid phase chemical methods permit the production of fine powders, whose average particle size is approximately 1 or 0.1 .mu.m with in addition a control of the morphology and a small size dispersion.
In chemical methods, a distinction must be made between two reaction types, namely decompositions and reductions.
Decomposition of carbonyl iron (by the liquid or gaseous process) to metallic iron powder gives spherical and relatively size-homogeneous powders of micronic or micronic dimensions, as a function of the operating conditions. The disadvantage of this method is that a large carbon monoxide volume is produced during the reaction. This method also does not permit the synthesis of iron-nickel or iron-cobalt polymetallic powders.
Among the chemical methods using the reduction phenomenon, reference is firstly made to reductions of solid iron oxides by various reducing agents. The Hoganas process uses coke for reducing the ground iron oxide. The powder obtained is formed from spongy particles, without any particular morphology and with an average size of a few dozen .mu.m. Other processes use gaseous hydrogen involving high reaction temperatures. By controlling the morphology of the starting oxide powder, certain authors have been able in special cases to control the morphology of the reduced metallic powder.
Liquid phase reductions take place under much more gentle temperature and pressure conditions and the morphology of the powders obtained is independent of that of the precursors, because the first stage of this type of reaction is generally the dissolving of the precursors in a solvent. The reducing agent and the dissolved metal salts react in solution to give the metal, which precipitates. The control of the metal particle size is dependent on the process used and requires a control or monitoring of the different reaction stages.
Borohydrides are used in aqueous solution to reduce iron or cobalt salts to fine powders of a controlled size, but the thus formed powders are metal-boron alloys.
The reduction of metal salts in an organic liquid medium by alkali metals such as lithium or sodium can give powders of iron and other metals, but the particles produced apparently have no specific morphology.
EP-A113 281 describes a process for reducing metallic compounds in a liquid polyol medium. This process is known as the "polyol process". In this case there is no addition of a supplementary reducing agent, the polyol serving both as the solvent and the reducing agent. This process makes it possible to obtain metallic powders, particularly of cobalt and nickel, with a control of the size and shape of the particles. However, it does not permit the obtaining of iron in metallic form, either pure, or associated with nickel or cobalt.